1. Field of the Invention
This invention relates to a photoelectronic conversion element using an amorphous semiconductor material, and more particularly to a target of an image pickup tube having a photoelectronic conversion part suitable for use as an imaging device used in a television camera or the like.
2. Description of the Prior Art
Amorphous silicon containing hydrogen (referred to hereinafter as "a-Si: H") has a high photoelectronic conversion efficiency and converts almost all of absorbed light into an electrical signal. It is an advantage of this a-Si: H that it can be doped with an impurity as in the case of crystalline semiconductors. It is another advantage of the a-Si: H that its film can be deposited at a low temperature on various substrates. Various devices making use of such advantages of the a-Si: H have been proposed hitherto. As a typical example of an imaging device using this a-Si: H, Japanese Patent Publication JP-B-57-046224 (1982) discloses an image pickup tube in which the a-Si: H is used to form a photoelectronic conversion film. The disclosed image pickup tube has various excellent features as follows: (1) the sensitivity for visible light is high; (2) the resolution is high; (3) it operates with a low lag and no sticking or no after image occurs after picking-up of still pictures for long period of time; and (4) it shows a high thermal stability.
However, the inventors found that, when a prior art image pickup tube using the a-Si: H was operated under acceleration of its scanning electron beam at a high voltage higher than about 700 to 1,000 volts, its operating characteristics changed, that is, its sensitivity was lowered, the dark current increased, etc.
The inventors considered that such a phenomenon might be similar to a phenomenon of lowering of the photoelectronic conversion efficiency attributable to irradiation with strong light, such as that also observed on a solar cell using the a-Si: H, and directed attention to soft X-rays generated due to collision of the scanning electron beam against the mesh-type accelerating electrode in the image pickup tube. Then, the inventors invented a method of suppressing progress of changes in the operating characteristics of such a tube by covering the surface of a mesh electrode with a material such as carbon or beryllium, as disclosed in JP-A-59-96639.
However, because no contrivence was applied to the a-Si: H photoconductive layer itself, in addition to the limitation of using a special mesh structure, the operating characteristics of the image pickup tube inevitably changed when the tube was incorporated in, for example, a monitoring camera continuously used for a very long period of time.